Researchers tap into quantum dots for optical communications

PORTLAND, Ore.  Semiconducting polymers embedded with lead-sulphide nanocrystals could produce a light source for integrated photonic chips, according to recent work at the University of Toronto.

The technique, producing infrared light at wavelengths used in communications systems, could be used to create photonic components orders of magnitude less expensive than current components, which can cost as much as $1,000.

Ted Sargent said his group in the university's Department of Electrical and Computer Engineering is also working on ways to build photonic crystal structures that could be deposited on silicon to create complete integrated photonic circuits. "We have found a way of making quantum dots of such a quality that they can produce light efficiently," said Sargent.

He collaborated with professor Gregory Scholes' group in the Department of Chemistry. Scholes said he is excited about the potential efficiency of the quantum dots. Simulations by his group have shown that by using the third-order optical nonlinear response, light intensities could be 30 times that of gallium-arsenide devices.

"Our work plays a role in the conversion of energy from electrical to opticalwe've shown a measurable efficiency at converting electrons into photons at wavelengths with which we can communicate," Lewis said. "This is where our work has relevanceat the interface between electronics and optics," he added.

Besides the nanocrystals' smaller size and, potentially, higher efficiency than today's electrical-to-optical converters, Sargent said his process for embedding them into a semiconducting polymer inherently simplifies and reduces the cost of optical chip manufacturing. Unlike traditional semiconductor fabrication techniques, the quantum dot and polymer fabrication techniques took place at normal pressures and relatively low temperatures.

By using simple thin-film techniques, Sargent sidesteps the exotic ovens and vacuum chambers needed to fabricate gallium-arsenide-style optics. As a result, his polymer/crystal nanocomposite produces light from electricity in a manner similar to an LED, but without the expense of traditional semiconductor fabrication.

"In a circuit sense, our quantum dots look a lot like forward-biased diodes. We apply a voltage bias, the current flows and the dots light up," he said. "Our objective here is efficiency, to have as much of that current as possible go into producing the light."

He added, "The great thing about dots, is that because they are so small, the energies that electrons are allowed to adopt inside them are determined by the diameter of the dotso that a big dot produces long wavelengths of light, whereas short wavelengths are produced by smaller dots."

Despite making quantum dots produce light, Sargent offered only educated guesses to explain the physical mechanism by which the quantum dots are excited. Electron and holes may be simultaneously tunneling to the dot, he suggested, or energy might be transferred without the physical movement of the electron-hole pair.